Processes for controlling afterburn in a reheater and for controlling loss of entrained solid particles in combustion product flue gas

ABSTRACT

Processes for controlling afterburn in a reheater and loss of entrained solid particles in reheater flue gas are provided. Carbonaceous biomass feedstock is pyrolyzed using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles. The spent heat transfer medium is introduced into a fluidizing dense bed. The combustible solid particles of the spent heat transfer medium are combusted forming combustion product flue gas in a dilute phase above the fluidizing dense bed. The combustion product flue gas comprises flue gas and solid particles entrained therein. The solid particles are separated from the combustion product flue gas to form separated solid particles. At least a portion of the separated solid particles are returned to the fluidizing dense bed.

FIELD OF THE INVENTION

The present invention generally relates to processes for controllingcombustion in a reheater of a pyrolysis system, and more particularlyrelates to a process for controlling afterburn in the reheater andcontrolling loss of entrained solid particles in combustion product fluegas during regeneration of a heat transfer medium.

DESCRIPTION OF RELATED ART

Pyrolysis is a thermal process during which solid carbonaceous biomassfeedstock, i.e., “biomass”, such as wood, agricultural wastes/residues,algae, forestry byproducts, cellulose and lignin, municipal waste,construction/demolition debris, or the like, is rapidly heated topyrolysis temperatures of about 300° C. to about 900° C. in the absenceof air using a pyrolysis reactor. Biomass may be pyrolyzed using variouspyrolysis methods, including the Rapid Thermal Process and catalyticpyrolysis. Under these conditions, solid, liquid, and gaseous pyrolysisproducts are formed. The gaseous pyrolysis products (“pyrolysis gases”)comprise a condensable portion (vapors) and a non-condensable portion.The solid pyrolysis products include combustible solid particlescontaining carbon referred to as “char”.

As known in the art, heat for the endothermic pyrolysis reaction isproduced in a reheater zone of a pyrolysis reactor or in a separatereheater (collectively referred to herein as a “reheater”) by combustingthe non-condensable pyrolysis gases and the combustible solid particlesproduced in the pyrolysis reaction. Heat is transferred from thereheater to the pyrolysis reactor by a “heat transfer medium.” The heattransfer medium typically comprises inert solid particles such as sand.In catalytic pyrolysis, catalytic solid particles may be used, insteadof or in addition to the inert solid particles, as the heat transfermedium. At the completion of pyrolysis, the combustible solid particleshave been mixed with the inert solid particles, the catalytic solidparticles if present, or both, forming spent heat transfer medium. Spentheat transfer medium has a reduced ability to transfer heat, and in thecase of catalytic solid particles, also a reduced catalytic activity. Torestore the heat transfer medium, the spent heat transfer medium iscontinuously transferred from the pyrolysis reactor to the reheaterafter separation from the pyrolysis gases. The spent heat transfermedium is regenerated in the reheater by combusting the combustiblesolid particles therein. The regenerated heat transfer medium is thenrecirculated to the pyrolysis reactor. During combustion, the carbon inthe combustible solid particles is converted to carbon dioxide. Removalof the carbon converts the combusted solid particles to ash. The buildupof ash in the reheater reduces the operating efficiency of the reheaterand reduces the volume available to combust “new” ash entering thereheater. Ash build-up in the reheater is thus undesirable, andtherefore its prompt removal from the reheater is desirable.

The heat transfer medium is maintained as a fluidized dense bed in alower portion of the reheater by the upward passage of anoxygen-containing regeneration gas stream through the fluidized densebed at a velocity of about 0.762 meters/second to about 0.9144meters/second (about 2.5 to about 3 feet per second). Combustion productflue gas is in a dilute phase in an upper portion of the reheater.During regeneration of the spent heat transfer medium in the reheater, aportion of the solid particles therein (combustible solid particles,inert solid particles and if present, catalytic solid particles) as wellas ash become entrained in the combustion product flue gas. The shortheight of the dense bed in the reheater and the size and densityproperties of the solid particles contribute to entrainment. The solidparticles, particularly the smaller and less dense combustible solidparticles and the ash, may be “blown” from the dense bed into the dilutephase because of the high superficial gas velocity of theoxygen-containing regeneration gas up through the dense bed.Unfortunately, if the combustible solid particles are not separated fromthe combustion product flue gas and returned to the fluidized dense bedof the reheater for combustion thereof, the entrained combustible solidparticles may cause “afterburning” of the combustible solid particles inthe dilute phase of the reheater or in downstream lines and equipment,rather than in the dense bed.

In addition to afterburning of the combustible solid particles,afterburning of the carbon monoxide in the oxygen-containingregeneration gas to CO₂ in the dilute phase may occur. Reheaterstypically are designed to operate so that substantially all of thecarbon monoxide (CO) in the oxygen-containing regeneration gas combuststo form carbon dioxide (CO₂), thereby imparting the heat of reaction tothe reheater. However, there may be incomplete combustion of the dilutephase flue gas CO to CO₂ or incomplete consumption of O₂ in the dilutephase. Either problem also gives rise to afterburning. Afterburning isexothermic, and either must be quenched by additional injection of theoxygen-containing regeneration gas, or the combustion product flue gasmust absorb the heat of combustion, which undesirably decreases theamount of heat transferred to the dense bed.

In addition to the afterburning problem caused by entrainment of thecombustible solid particles, a portion of the hot regenerated inert andcatalytic solid particles may be lost if not separated from thecombustion product flue gas and returned to the dense bed forrecirculation as the heat transfer medium or as a catalyst (in the caseof the catalytic solids). Conventional regeneration methods have reliedupon a single stage of gas-solid separators downstream of and outsidethe reheater to separate the entrained solid particles from thecombustion product flue gas. However, the capacity of such separators isoften exceeded and such outside separators cannot remove ash from thereheater promptly after combusting the carbon in the combustible solidparticles and cannot return the solid particles to the dense bed whilethe solid particles are still in the reheater. Further attempts toprevent loss of the inert solid particles, catalytic solid particles, orboth have included reducing the superficial gas velocity of theoxygen-containing regeneration gas below an optimized superficial gasvelocity and, in the case of the inert solids, increasing their particlesize and density to resist entrainment in the combustion product fluegas. However, these changes have not entirely prevented loss of suchsolid particles in the combustion product flue gas. Such loss increasesproduction costs and lowers throughput of regenerated heat transfermedium to the pyrolysis reactor.

Accordingly, it is desirable to provide processes for controllingafterburn in a reheater and loss of entrained solid particles in thecombustion product flue gas during regeneration of the heat transfermedium. It is also desirable to remove ash from the reheater promptlyupon its formation and optimize the superficial gas velocity and sizeand density properties of the solid particles for regeneration.Furthermore, other desirable features and characteristics of the presentinvention will become apparent from the subsequent detailed descriptionof the invention and the appended claims, taken in conjunction with theaccompanying drawings and this background of the invention.

SUMMARY OF THE INVENTION

Processes are provided for controlling afterburn in a reheater and lossof entrained solid particles from reheater flue gas. In accordance withone exemplary embodiment, the process for controlling afterburn and lossof entrained solid particles comprises pyrolyzing carbonaceous biomassfeedstock using a heat transfer medium forming pyrolysis products and aspent heat transfer medium comprising combustible solid particles. Thespent heat transfer medium is introduced into a fluidizing dense bed.The combustible solid particles of the spent heat transfer medium arecombusted forming combustion product flue gas in a dilute phase abovethe fluidizing dense bed. The combustion product flue gas comprisesproduct flue gas and solid particles entrained therein. The solidparticles are separated from the combustion product flue gas to formseparated solid particles. At least a portion of the separated solidparticles are returned to the fluidizing dense bed.

Processes are provided for controlling afterburn in a reheater and lossof entrained solid particles from reheater flue gas in accordance withyet another exemplary embodiment of the present invention. The processcomprises introducing spent heat transfer medium comprising combustiblesolid particles mixed with inert solid particles, catalytic solidparticles, or both, into an oxygen-containing regeneration gas upwardlypassing through a fluidized dense bed of heat transfer medium in areheater at a temperature between about 300° C. to about 900° C.Combustion product flue gas is produced having at least a portion of thecombustible solid particles mixed with the inert solid particles, thecatalytic solid particles, or both entrained therein. The combustionproduct flue gas is passed through a flue gas-solids separator disposedin the reheater to produce substantially solids-free flue gas andseparated combustible solid particles mixed with separated inert solidparticles, separated catalytic solid particles, or both. At least aportion of the separated combustible solid particles mixed with theseparated inert solid particles, the separated catalytic solidparticles, or both, are passed to the fluidized dense bed.

Processes are provided for controlling afterburn in a reheater and lossof entrained solid particles from reheater flue gas in accordance withyet another exemplary embodiment of the present invention. The processcomprises discharging the combustion product flue gas with entrainedsolid particles from a fluidized dense bed of a reheater into a dilutevapor phase in an upper portion of the reheater. Centrifugally separatedsolids are recovered in the fluidized dense bed in a bottom portion ofthe reheater from a flue gas-solids separator disposed in the reheater.Substantially solids-free flue gas separated from the entrained solidparticles is passed through a flue gas transfer line in opencommunication with an external cyclone separator. Residual entrainedsolid particles are further separated from the substantially solids-freeflue gas before effecting recovery of product flue gas from the externalcyclone separator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 is a flow chart of a process for controlling afterburn in areheater and loss of entrained solid particles from the combustionproduct flue gas, according to exemplary embodiments of the presentinvention;

FIG. 2 is a schematic block diagram of an exemplary overall pyrolysisprocess flow, in accordance with exemplary embodiments of the presentinvention;

FIG. 3 is a cross-sectional view of a reheater having a cycloneseparator disposed therein as used in the process of FIG. 1, accordingto exemplary embodiments of the present invention; and

FIG. 4 is a cross-sectional view of a reheater having a vortex separatordisposed therein as used in the process of FIG. 1, according toexemplary embodiments of the present invention.

DETAILED DESCRIPTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Various exemplary embodiments of the present invention are directed toprocesses for controlling afterburn and loss of entrained solidparticles in combustion product flue gas during regeneration of a heattransfer medium in a reheater of a pyrolysis system. The “reheater” maybe a reheater zone of a pyrolysis reactor or a reheater separate fromthe pyrolysis reactor. The reheater is equipped with an internalgas-solids separator, such as a cyclone separator, a vortex separator,or both, as hereinafter described. Controlling afterburn and loss ofentrained solid particles increases the amount of heat transferred tothe reheater dense bed for regeneration of the heat transfer medium andalso preserves the inert solid particles, the catalytic solid particles,or both, for recycling to the pyrolysis reactor, thereby increasingthroughput to the pyrolysis reactor.

FIG. 1 is a process for controlling afterburn and loss of entrainedsolid particles from combustion product flue gas during regeneration ofa spent heat transfer medium in accordance with an exemplary embodimentof the present invention. FIG. 2 is an exemplary embodiment of apyrolysis system 5 that utilizes the process of FIG. 1. Referring toFIGS. 1 and 2, the process 10 begins by pyrolyzing carbonaceous biomassfeedstock (hereinafter “biomass”) 15 in a pyrolysis reactor 20 using aheat transfer medium and forming pyrolysis products and a spent heattransfer medium (step 100). As noted previously, the pyrolysis productscomprise solid, liquid, and gaseous pyrolysis products. The gaseouspyrolysis products comprise a condensable portion (vapors) and anon-condensable portion. The condensable portion may be condensed intoliquid biomass-derived pyrolysis oil. The solid pyrolysis productsinclude combustible solid particles containing carbon (also referred toherein as “char”). The heat transfer medium comprises inert solidparticles, such as sand, catalytic solid particles, or both. The heattransfer medium leaving the pyrolysis reactor is said to be “spent”,because it contains combustible carbon-containing solids. The spent heattransfer medium leaving the pyrolysis reactor is entrained in thegaseous pyrolysis products (“pyrolysis gases”). The pyrolysis gases withentrained spent heat transfer medium are referred to in FIG. 2 with thereference number 35. The pyrolysis gases with entrained spent heattransfer medium are transferred from the pyrolysis reactor to apyrolysis gas-solid separator 30 for separation into pyrolysis gases 45and spent heat transfer medium 55.

Next, in accordance with an exemplary embodiment, and as shown in FIGS.1 and 2, the process continues by introducing the spent heat transfermedium 55 from the pyrolysis gas-solid separator 30 into a fluidizeddense bed 56 in a reheater 40 (step 200). An exemplary reheater (shownin FIGS. 3 and 4) comprises a large vertical substantially cylindricalvessel 110 wherein the heat transfer medium is maintained as thefluidized dense bed 56 in the reheater by the upward passagetherethrough of an oxygen-containing regeneration gas stream 115,preferably air, which also agitates the heat transfer medium within thefluidized dense bed. The oxygen-containing regeneration gas stream flowsupward through the spent heat transfer medium at a superficial gasvelocity above the minimum velocity required to fluidize the solidparticles of the heat transfer medium. The superficial gas velocity(V_(fs)) of the oxygen-containing regeneration gas may be calculatedusing the following equation:

V _(fs)=[volume flow of gas]/[cross sectional area of pipe (conduit)]

wherein subscript “s” denotes superficial and subscript “f” refers tothe fluid. The fraction of vessel cross-sectional area available for theflow of gas is usually assumed to be equal to the volume fractionoccupied by the gas, that is, the voidage or void fraction ε. Thesuperficial gas velocity should be optimized to avoid operating thefluidized dense bed in a “slugging flow regime”, i.e., it is desirableto operate the reheater at a superficial gas velocity above thesuperficial gas velocity at which the entrainment rate of solidparticles is high, in order to reduce the diameter of the vessel. Aspreviously noted, however, an optimized superficial gas velocity may“blow” the solid particles of the heat transfer medium (along withcombustible solid particles as hereinafter described) from the fluidizeddense bed 56 in a lower portion of the reheater vessel into a dilutevapor phase 65 in an upper portion of the reheater vessel above thefluidized dense bed of heat transfer medium. The oxygen-containingregeneration gas is distributed in the reheater through a reheaterdistributor 120. The spent heat transfer medium 55 is introduced intothe reheater through an inlet conduit 125 and passed (carried) as asuspension by the oxygen-containing regeneration gas through thefluidized dense bed 56 of heat transfer medium in the reheater.

Referring to FIGS. 1 and 2, at least a portion of the combustible solidparticles of the spent heat transfer medium are combusted using thestream of oxygen-containing regeneration gas (step 250). Heat from thecombustion is transferred to the heat transfer medium in the fluidizeddense bed and combustion product flue gas 70 is produced. The oxygenprovided by the oxygen-containing regeneration gas stream may compriseat least the stoichiometric amount of oxygen needed for substantiallycomplete combustion of the combustible solid particles, or an excessthereof. Alternatively, there may be additional oxidant streams if lessthan the stoichiometric amount of oxygen is provided by theoxygen-containing regeneration gas stream. Combustion raises thetemperature of the dense bed material (i.e., the heat transfer medium)to the operating conditions needed in the pyrolysis reactor 20, i.e., toabout 300° C. to about 900° C. The reheater is typically maintained at atemperature range of about 400° C. to about 1000° C.

The combustion product flue gas 70 is discharged from the fluidizeddense bed 56 into the dilute vapor phase 65 in the upper portion of thereheater. The combustion product flue gas contains gases arising fromthe combustion of the combustible solid particles such as carbondioxide, carbon monoxide from the oxygen-containing regeneration gasstream, inert gases such as nitrogen from air, and unreacted oxygen. Thecombustion product flue gas also contains entrained solid particlesincluding non-combusted combustible solid particles 75 and hot dense bedmaterial comprising hot regenerated inert solid particles 80, hotregenerated catalytic solid particles 85, or a combination thereof. Thecombustion product flue gas also contains ash particles.

The process 10 continues by separating the solid particles from thecombustion product flue gas and returning a portion thereof to thefluidized dense bed 56 (step 300). In one exemplary embodiment, aportion of the solid particles are separated from the combustion productflue gas forming substantially solids-free flue gas 90 using a fluegas-solids separator 50. In another exemplary embodiment, the fluegas-solids separator is disposed in the reheater, as illustrated in FIG.2. The substantially solids-free flue gas may contain residualcombustible solid particles and residual ash particles as theseparticles are generally smaller (on average) than the inert solidparticles and the catalytic solid particles and therefore not as easilyseparated from the flue gas in the flue gas-solids separator 50. Thatthe substantially solids-free flue gas may contain residual ashparticles enables the ash particles to escape the reheater confines,thus substantially preventing ash build-up in the reheater.

A portion of the separated combustible solid particles 75 are returnedto the fluidized dense bed for combustion, which minimizes combustion(i.e., “afterburning”) of the combustible solid particles in the dilutevapor phase or downstream therefrom. The separated hot regenerated inertsolid particles 80, separated hot regenerated catalytic solid particles85, or both, are returned to the dense bed 56 where they are withdrawnand returned to the pyrolysis reactor through outlet conduit 130 (FIGS.3 and 4) for further usage in pyrolyzing carbonaceous biomass feedstock,as illustrated by arrow 25 in FIGS. 2-4. Outlet conduit 130 includes avalve 135 used to control the solids flow. A slide valve, for example,may be used. The separated hot regenerated inert solid particles 80 maybe returned to the pyrolysis reactor for further usage as the heattransfer medium. The separated hot regenerated catalytic solid particles85 may be returned to the pyrolysis reactor for usage as the heattransfer medium, a pyrolysis catalyst, or both.

The flue gas-solids separator 50 allows greater contact between the heattransfer medium and the combustible solid particles, resulting in ahigher percentage of the heat released from combustion to be transferredto the heat transfer medium while still in the reheater. The optimizedsuperficial gas velocity may be maintained and smaller, more fluidizableheat transfer medium may advantageously be used without significantconcern that the solid particles will “blow” into the dilute vapor phaseand be irretrievably lost. Smaller heat transfer medium particlesincrease the surface area for heat transfer making the heat transfermedium more fluidizable.

Referring to FIG. 3, in one embodiment, the flue gas-solids separator 50comprises a cyclone separator 50 a, which centrifugally separates theentrained solid particles from the combustion product flue gas. WhileFIG. 3 illustrates two cyclone separators in parallel, one cycloneseparator may be used or more than two cyclone separators could beemployed in the same parallel arrangement as illustrated, in a seriesflow arrangement, or in a different flow arrangement as the volume andloading of the combustion product flue gas vapor stream and the desireddegree of separation dictate. An exemplary cyclone separator 50 a, asillustrated, comprises an upper, generally cylindrical barrel portion 51having a first wall 52, and a lower, generally conical portion 53terminating in a solids outlet 54 with a diameter smaller than thebarrel portion. The lower open end of the barrel portion 51 and theconical portion 53 at its wider diameter end are adjoined and/or areintegral and together define a separation chamber. A generallycylindrical, solids discharge dipleg 57 has an upper end in opencommunication with the solids outlet 54 and a lower end 58 wherebyseparated solids can be removed from the cyclone separator. The lowerend 58 of the solids discharge dipleg includes sealing means. Thepurpose of the sealing means is to substantially ensure that the solidsdischarge dipleg is sealed against the possibility of combustion productflue gas entering into its interior, which would cause a loss inseparation efficiency. In a preferred embodiment, the sealing meanscomprises immersing the lower end of the solids discharge dipleg 57 inthe fluidized dense bed of the reheater, i.e., below a top surface 59 ofthe fluidized dense bed.

In another embodiment, the sealing means comprises a sealing device 61connected to the lower end of the solids discharge dipleg. Sealingdevices may be of several types, such as flapper valves, trickle valves,or the like. An exemplary trickle valve is shown in FIG. 3. While FIG. 3illustrates each of the cyclone separators having different sealingmeans, it is to be appreciated that the sealing means at the lower endsof the solids discharge diplegs may be the same for each cycloneseparator. In operation, the combustion product flue gas 70 in thedilute vapor phase 65 enters a gas inlet 62 of each of the cycloneseparators and is introduced tangentially into the barrel portion 51.The solid particles from the combustion product flue gas, because oftheir inertia, move toward the walls of the cyclone separator and spiraldownwardly toward the separation chamber, being ultimately dischargedthrough the solids discharge dipleg(s) 57 into or onto the dense bed inthe reheater.

Referring to FIGS. 2 and 3, the substantially solids-free flue gas 90from cyclone separator 50 a passes upwardly through a gas outlet tube 63and is discharged though an upper end into a plenum 64. It is thenvented or otherwise removed from the reheater via flue gas line 170 andis passed to a conventional external cyclone separator 60 for removal ofany residual entrained solid particles 95, such as combustible solids,sand, ash, or catalytic solids producing product flue gas 105. The sandand ash may be removed from the external cyclone separator for disposal.Catalytic solid particles may be recirculated to the reheater for reuse,as illustrated by arrow 26 in FIG. 2.

In another embodiment, as shown in FIG. 4, the flue gas-solids separator50 comprises a vortex separator 50 b (also known as a swirlconcentrator) disposed in the reheater. One or more vortex separatorsmay be disposed in the reheater and one or more vortex separators may beused in combination with cyclone separators. Exemplary vortex separatorsfor use in process 10 are described, for example, in U.S. Pat. No.4,482,451 and U.S. Pat. No. 5,584,985 by the same named assignee, thecontents of which are incorporated herein by reference in theirentirety. Generally, the vortex separator 50 b comprises a centralconduit in the form of a riser 140 which extends upwardly from a lowerportion of the reheater. The central conduit or riser 140 preferably hasa vertical orientation within the reheater and may extend upwardly fromthe bottom of the reheater vessel 110. Riser 140 terminates in an upperportion of the reheater vessel 110 with a curved conduit in the form ofan arm 145. The arm 145 discharges the combustion product flue gas 70into the dilute vapor phase 65 of the reheater. The tangential dischargeof the combustion product flue gas from a discharge opening 150 of thearm 145 produces a centrifugal (swirling helical) pattern about theinterior of the vessel 110 below the discharge opening. Centripetalacceleration associated with the helical motion forces the separated hotregenerated solid particles 75, 80, 85 to the inside walls of the vessel110. The separated hot regenerated solid particles collect in the bottomof the separation vessel. The separated hot regenerated solid particlesexit the bottom of the separation vessel through discharge conduits 160into the fluidized dense bed 56 in the reheater. The substantiallysolids-free flue gas 90 from the vortex separator 50 b passes upwardlythrough a gas outlet 155 to the flue gas line 170 where it is vented orotherwise removed from the reheater and passed to the external cycloneseparator 60 for removal of any residual entrained solid particles 95,such as combustible solids, sand, ash, and/or catalytic solids producingproduct flue gas 105. The sand and ash may be removed from the externalcyclone separator 60 for disposal. Catalytic solid particles may berecirculated to the reheater for reuse, as illustrated by arrow 26 inFIG. 2.

From the foregoing, it is to be appreciated that the processes inaccordance with the exemplary embodiments as described herein helpcontrol afterburn and loss of entrained solid particles from thecombustion product flue gas. Separating the entrained combustible solidparticles from the combustion product flue gas and returning them to thedense bed helps control afterburn in the dilute phase, therebyincreasing the amount of heat transferred to the reheater dense bed forregeneration of the heat transfer medium. Separating the entrained inertsolid particles, catalytic solid particles, or both of the heat transfermedium from the combustion product flue gas and returning the solidparticles to the dense bed helps preserve such solid particles in thepyrolysis system. Production costs are therefore reduced and there is anincreased throughput of regenerated heat transfer medium to thepyrolysis reactor. Similarly, passing the combustible solid particles tothe flue gas-solids separator while still in the reheater and in contactwith the inert solid particles, catalytic solid particles, or both, alsoincreases the amount of heat transferred to the reheater dense bed. Inaddition, as the entrained solid particles are returned to the densebed, efforts to resist entrainment such as reducing the superficial gasvelocity below an optimized velocity and disadvantageously increasingthe size and density of the solid particles of the heat transfer mediummay no longer be necessary.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

1. A process for pyrolysis of a carbonaceous biomass feedstock, theprocess comprising the steps of: pyrolyzing carbonaceous biomassfeedstock using a heat transfer medium forming pyrolysis products and aspent heat transfer medium comprising combustible solid particles;introducing the spent heat transfer medium into a fluidized dense bedand combusting the combustible solid particles of the spent heattransfer medium forming combustion product flue gas in a dilute phaseabove the fluidized dense bed, the combustion product flue gascomprising flue gas and solid particles entrained therein; andseparating the solid particles from the combustion product flue gas toform separated solid particles and returning at least a portion of theseparated solid particles to the fluidized dense bed.
 2. The process ofclaim 1, wherein the step of introducing spent heat transfer mediumcomprises introducing spent heat transfer medium comprising combustiblesolid particles mixed with inert solid particles, catalytic solidparticles, or both.
 3. The process of claim 1, wherein the step ofintroducing spent heat transfer medium comprises introducing the spentheat transfer medium into a fluidized dense bed maintained by anoxygen-containing regeneration gas stream at a superficial gas velocityabove the minimum velocity to fluidize the fluidized dense bed.
 4. Theprocess of claim 1, wherein the step of separating is performed using aflue gas-solids separator.
 5. The process of claim 4, wherein thefluidized dense bed and the flue gas-solids separator are in a reheater.6. The process of claim 1, wherein the step of separating is performedwith a cyclone separator, a vortex separator, or a combination thereof.7. The process of claim 6, wherein the step of separating comprisesseparating with the cyclone separator having a lower end of a solidsdischarge dipleg immersed below a top surface of the fluidized densebed.
 8. The process of claim 1, wherein the step of separating comprisesseparating combustible solid particles to form separated combustiblesolid particles and returning at least a portion of the separatedcombustible solid particles to the fluidized dense bed for combustiontherein.
 9. The process of claim 1, wherein the step of separatingcomprises separating inert solid particles, catalytic solid particles,or both, to form separated inert solid particles, separated catalyticsolid particles, or both, and returning the separated inert solidparticles, separated catalytic solid particles, or both, to thefluidizing dense bed.
 10. A process for controlling afterburn in areheater and loss of entrained solid particles from combustion productflue gas, the process comprising the steps of: introducing spent heattransfer medium comprising combustible solid particles mixed with inertsolid particles, catalytic solid particles, or both, into anoxygen-containing regeneration gas upwardly passing through a fluidizeddense bed of heat transfer medium in a reheater at a temperature betweenabout 300° C. to about 900° C., producing combustion product flue gashaving at least a portion of the combustible solid particles mixed withthe inert solid particles, the catalytic solid particles, or bothentrained therein; passing the combustion product flue gas through aflue gas-solids separator disposed in the reheater to producesubstantially solids-free flue gas and separated combustible solidparticles mixed with separated inert solid particles, separatedcatalytic solid particles, or both; and passing at least a portion ofthe separated combustible solid particles mixed with the separated inertsolid particles, the separated catalytic solid particles, or both, tothe fluidized dense bed.
 11. The process of claim 10, wherein the stepof passing at least a portion of the separated combustible solidparticles comprises passing at least a portion of the separatedcombustible solid particles to the fluidized dense bed and combustingthe separated combustible solid particles.
 12. The process of claim 10,further comprising the step of recirculating the separated inert solidparticles, the separated catalytic solid particles, or both, from thefluidized dense bed for use as the heat transfer medium in pyrolyzingcarbonaceous biomass feedstock.
 13. The process of claim 10, furthercomprising the step of recirculating the separated catalytic solidparticles from the fluidized dense bed for use as a catalyst inpyrolyzing carbonaceous biomass feedstock.
 14. The process of claim 10,wherein the step of passing the combustion product flue gas through aflue gas-solids separator comprises passing the combustion product fluegas through a cyclone separator, a vortex separator, or a combinationthereof.
 15. The process of claim 14, wherein the step of passing thecombustion product flue gas through a flue gas-solids separatorcomprises passing the combustion product flue gas through the cycloneseparator having a lower end of a solids discharge dipleg immersed belowa top surface of the fluidized dense bed.
 16. A process for controllingafterburn in a reheater and loss of entrained solid particles incombustion product flue gas, the process comprising the steps of:discharging the combustion product flue gas with entrained solidparticles from a fluidized dense bed of a reheater into a dilute vaporphase in an upper portion of the reheater; recovering centrifugallyseparated solid particles in the fluidized dense bed in a bottom portionof the reheater from a flue gas-solids separator disposed in thereheater; and passing a substantially solids-free flue gas separatedfrom the entrained solid particles through a flue gas transfer line inopen communication with an external cyclone separator wherein residualentrained solid particles are further separated from the substantiallysolids-free flue gas before effecting recovery of product flue gas fromthe external cyclone separator.
 17. The process of claim 16, wherein thestep of recovering centrifugally separated solid particles comprisesrecovering separated combustible solid particles and combusting at leasta portion of the separated combustible solid particles in the fluidizeddense bed.
 18. The process of claim 16, wherein the step of recoveringcentrifugally separated solid particles comprises recoveringcentrifugally separated inert solid particles, centrifugally separatedcatalytic solid particles, or both.
 19. The process of claim 18, whereinthe step of recovering centrifugally separated solid particles comprisesrecirculating the centrifugally separated catalytic solid particles to apyrolysis reactor.
 20. The process of claim 16, wherein the step ofpassing a substantially solids-free flue gas comprises recovering thefurther separated residual entrained solid particles.